1. Field of the Invention
The invention relates to the location of patterns within an image, and more particularly, the invention relates to the location of patterns within an image utilizing an adaptive threshold.
2. Description of the Prior Art
Pattern location systems for locating predetermined patterns within an image are known in the art. A type of location system is shown in U.S. Pat. No. 3,776,447 issued to Adams et al on Dec. 4, 1973 and entitled "Automatic Semiconductor Bonding Machine". In Adams, an object is located and that information is utilized to position a tool for operation on the object. The object in Adams is a semiconductor integrated circuit, and the tool is the bonding head of wire bonder.
In general, a pattern location system determines the location of a pattern within a image by comparing portions of the image with a reference pattern. The image is observed by a video camera. The video camera is positioned to be incident to light reflected from the object. The camera scans the light it receives and produces an analog output which correspondes to the image of the object present in the reflected light. Typically the video camera produces a raster scan of the light reflected from the object. The analog output produced by the video camera has an amplitude varying in proportion to the magnitude of the reflected light at the corresponding location within the image. The light reflected from the object is repeatedly electronically scanned by the video camera.
The analog output of the video camera is digitized and the digital output produced is usually stored in a memory. When the analog output is digitized, a certain amplitude is selected. The analog output is sampled at a known rate. If the amplitude of the analog output when sampled exceeds the certain amplitude, a digital signal of one level is generated. If the amplitude of the analog output when sampled is less than or equal to the certain amplitude, a digital signal of another level is generated. As the digital signals are generated they form a digital output which is stored into a video memory. The storage of the digital output is such that the relationship between the individual digital and the corresponding location of the digital signal within the image is known and maintained.
The digital image within the video memory is compared with a stored reference image. The comparison is usually repeated as the image within the video memory is shifted relative to the reference image. The best match between the reference image, which is the predetermined pattern, is found by the repeated comparisons. The location of the pattern within the video image is noted and the information transferred to an alignment apparatus. The align apparatus is then actuated and the position of the object relative to a tool is altered. An X-Y table can be utilized for this purpose. The motors attached to the table can be actuated and the relative position of the object and tool altered. The motors are usually driven by a displacement device which utilizes the location of the pattern within the stored image to determine the distance and direction of the relative movement necessary to bring the predetermined pattern on the object to a desired position relative to the tool.
None of the prior constructions shows an apparatus and method of pattern location utilizing an adaptive threshold for determining if the digital output produced from the analog output of a video camera is of one digital level or another digital level.